Hereinafter, a general frame structure used in a radio access system will be described.
FIG. 1 is a view showing a frame structure used in a wideband radio access system (e.g., the IEEE 802.16 system).
Referring to FIG. 1, a horizontal axis of a frame indicates an orthogonal frequency division multiplexing access (OFDMA) symbol as a time unit, and a vertical axis of the frame indicates a logical number of a sub-channel as a frequency unit. In FIG. 1, one frame is divided into data sequence channels during a predetermined time period by physical characteristics. That is, one frame includes one downlink (DL) sub-frame and one uplink (UL) sub-frame. The downlink sub-frame and the uplink sub-frame are distinguished from each other by a transmit transition gap (TTG), and frames are distinguished from each other by a receive transition gap (RTG).
At this time, the DL sub-frame may include one preamble, a frame control header (FCH), a DL-MAP, a UL-MAP, and one or more data bursts. In addition, the UL sub-frame may include one or more UL data bursts and a ranging sub-channel.
In FIG. 1, the preamble is specific sequence data located at a first symbol of every frame and is used to perform synchronization of a mobile station with a base station or estimation of a channel. The FCH is used to provide channel allocation information and channel code information associated with the DL-MAP. The DL-MAP/UL-MAP is a media access control (MAC) message used for informing a mobile station of channel resource allocation in downlink/uplink. In addition, the data burst indicates the unit of data which is transmitted from a base station to a mobile station or from a mobile station to a base station.
A downlink channel descriptor (DCD) which may be used in FIG. 1 indicates an MAC message indicating the physical characteristics of a DL channel and an uplink channel descriptor (UCD) indicates an MAC message indicating the physical characteristics of a UL channel.
In downlink, referring to FIG. 1, the mobile station detects the preamble transmitted from the base station and performs the synchronization with the base station. Thereafter, the DL-MAP may be decoded using information acquired from the FCH. The base station may transmit scheduling information for DL or UL resource allocation to the mobile station in each frame (e.g., 5 ms) using the DL-MAP or UL-MAP message.
Since the DL-MAP/UL-MAP message shown in FIG. 1 is transmitted with a modulation coding scheme (MCS) level which can be received by every mobile station, unnecessary map message overhead may occur. For example, since mobile stations located in the vicinity of the base station have good channel statuses, a high MCS level (e.g., QPSK 1/2) may be used for encoding or decoding the message. However, the base station encodes the map message with a low MCS level (e.g., QPSK 1/12) and transmits the map message, for mobile stations which are located at the edges of its cell, without considering the channel status. Accordingly, since each mobile station always receives the message encoded with the same MCS level regardless of the channel status, unnecessary map message overhead may occur.
FIG. 2 is a view showing an example of hybrid automatic repeat request (HARQ) control signal delay at the time of transmission of DL data used generally.
Referring to FIG. 2, in any frame (e.g., an Nth frame) of a wideband radio access system (e.g., WiMAX), a base station may transmit a DL-MAP to a mobile station and inform the mobile station of DL burst information of a current frame. The mobile station may receive a DL data burst from the base station in an Nth frame (S0101).
In addition, the base station may transmit a UL-MAP to the mobile station in the Nth frame and inform the mobile station of UL channel information for transmitting a control signal (e.g., an acknowledgement (ACK) signal). Accordingly, generally, if the HARQ is applied, the mobile station may transmit an ACK/NACK signal of a DL data burst to the base station in an N+1th frame (S0102).
In FIG. 2, HARQ ACK delay may be generated by at least one frame. In addition, if the NACK signal is generated, retransmission delay may be increased by the processing delay of the base station. That is, the frame structure (e.g., the IEEE 802.16e system) used generally has fixed ACK delay with respect to the DL burst.
In a general radio access system, as described with reference to FIG. 2, the processing delay, the transmission delay or the like may occur. Accordingly, in order to prevent the transmission delay or the processing delay, there is a need for developing a new wideband radio access system. At this time, the new wideband radio access system uses the frame structure of a general radio access system and a new frame structure.
In the general radio access system, a transmit time interval (TTI) is set in the unit of frames. Accordingly, if the concept of the existing TTI is applied to the new radio access system without change, the base station should transmit scheduling information for downlink traffic to the mobile station via a sub-map in every sub-frame. In this case, a probability that a radio resource may be wasted is high.